Intermittent fasting (IF) has become one of the most talked-about approaches to eating in recent years. And unlike many diet trends, it has actual science behind it.
People use it to lose weight, improve metabolic health, and simplify their daily routine. Research suggests it can benefit both body and brain — and possibly help you live longer.1
This guide covers everything you need to know to get started.
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What is intermittent fasting (IF)?
Intermittent fasting is an eating pattern that cycles between periods of fasting and eating. It doesn’t dictate what you eat — it focuses on when you eat.
Think of it less as a diet and more as an eating schedule.
The most common approaches involve fasting for 16 hours a day or going a full 24 hours without food twice a week.
Fasting itself is nothing new. Our hunter-gatherer ancestors didn’t have 24/7 access to food. They regularly went extended periods without eating, and our bodies evolved to handle that just fine. In many ways, occasional fasting is more natural than eating three-plus meals every day.
Fasting also has a long history in religious practice — Islam, Christianity, Judaism, and Buddhism all include fasting traditions.
Summary: Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It focuses on meal timing rather than food choices.
Intermittent fasting methods
There are several ways to do intermittent fasting. All of them split your day or week into eating and fasting windows.
During fasting periods, you eat very little or nothing at all.
Here are the most popular approaches:
- The 16/8 method: Also called the Leangains approach. You skip breakfast and eat within an 8-hour window — say, 1 p.m. to 9 p.m. — then fast for the remaining 16 hours. Learn more about 16/8 intermittent fasting.
- Eat-stop-eat: You fast for a full 24 hours once or twice a week. For example, from dinner one evening to dinner the next.
- The 5:2 diet: You eat normally five days a week and restrict calories to 500–600 on two nonconsecutive days. Read our full guide on the 5:2 diet.
All these methods reduce your overall calorie intake through the fasting periods. As long as you don’t compensate by overeating during your eating windows, weight loss typically follows.
The 16/8 method tends to be the most popular because it’s the simplest and easiest to stick with.
Summary: There are several intermittent fasting methods. The 16/8 method, eat-stop-eat, and the 5:2 diet are the most widely used.
Suggested read: What breaks a fast? Foods, drinks, and supplements
How intermittent fasting affects your cells and hormones
When you fast, your body undergoes changes at the cellular and molecular level. Hormone levels shift to make stored body fat more accessible, and your cells ramp up repair processes.1
Here’s what happens during a fast:
- Human growth hormone (HGH): Growth hormone levels rise significantly during fasting. Higher HGH supports fat loss and muscle preservation.2
- Insulin: Insulin sensitivity improves and insulin levels drop, making it easier for your body to tap into fat stores. Learn more about insulin and insulin resistance.
- Cellular repair: Your cells activate autophagy — a cleanup process where they break down and recycle damaged proteins and cellular components.1
- Gene expression: Fasting triggers changes in gene activity related to longevity and disease protection.
These shifts in hormones and cellular function are what drive many of the health benefits associated with intermittent fasting. Read more about how intermittent fasting affects your metabolism.
Summary: When you fast, growth hormone levels rise, insulin drops, and your cells initiate repair processes like autophagy. These changes underpin the health benefits of IF.
Intermittent fasting for weight loss
Weight loss is the number one reason people try intermittent fasting — and it works through two mechanisms.
First, eating fewer meals naturally leads to consuming fewer calories.
Second, fasting changes your hormones in ways that support fat burning. Lower insulin, higher growth hormone, and increased norepinephrine all help your body break down fat and use it for energy.1
Research backs this up. An umbrella review of meta-analyses published in JAMA Network Open found that intermittent fasting — especially modified alternate-day fasting — led to meaningful reductions in BMI, body weight, fat mass, and several cardiometabolic risk factors.3
A separate systematic review found that intermittent fasting produced roughly 4 kg of weight loss compared to no treatment, and performed similarly to continuous calorie restriction.4
Another meta-analysis found that IF may even have a slight edge over standard calorie restriction for weight loss.5
A 2022 randomized clinical trial showed that early time-restricted eating (eating from 7 a.m. to 3 p.m.) led to 2.3 kg more weight loss than a standard eating window, along with improved blood pressure and mood.6
It’s worth noting: the effectiveness of IF still comes down to calories. If you overeat during your eating windows, you won’t see results.
Summary: Intermittent fasting helps with weight loss by reducing calorie intake and shifting hormones to promote fat burning. Multiple meta-analyses confirm its effectiveness.
Health benefits of intermittent fasting
The benefits of intermittent fasting go well beyond weight loss. Here’s what the research shows:1 3
- Weight loss: IF reduces body weight and belly fat without requiring you to consciously count calories.
- Insulin resistance: IF can lower fasting blood sugar and improve insulin sensitivity, which may protect against type 2 diabetes.3
- Inflammation: Some studies show reduced markers of inflammation — a driver behind many chronic diseases.
- Heart health: IF may improve LDL cholesterol, triglycerides, blood sugar, and blood pressure — all risk factors for heart disease.3
- Cancer: Animal studies suggest IF may slow tumor growth, though human evidence is still limited.
- Brain health: Fasting increases brain-derived neurotrophic factor (BDNF) and may support the growth of new nerve cells. It might also protect against Alzheimer’s disease.1
- Anti-aging: In animal studies, fasting extended lifespan in rats by 36–83%.
Keep in mind: much of this research is still evolving. Many studies were small, short-term, or conducted on animals. We need more large-scale, long-term human trials to confirm these findings.
Read our in-depth article on the health benefits of intermittent fasting.
Summary: IF offers benefits beyond weight loss, including improved insulin sensitivity, reduced inflammation, better heart health markers, and potential brain-protective effects.
Suggested read: 8 Science-Backed Health Benefits of Fasting
Intermittent fasting makes healthy eating simpler
One of the most underrated perks of intermittent fasting? It simplifies your life.
Fewer meals means less time spent planning, cooking, and cleaning up. You don’t have to think about breakfast. You prepare fewer meals. Your grocery list gets shorter.
For people who find complex meal plans exhausting, IF removes a lot of the mental load around food. That’s a big reason it sticks for many people.
Summary: IF simplifies daily eating by reducing the number of meals you need to plan and prepare.
Who should be careful or avoid intermittent fasting?
Intermittent fasting isn’t right for everyone.
If you’re underweight or have a history of eating disorders, talk to a healthcare professional before trying any form of fasting. In these cases, IF can do more harm than good.
Is intermittent fasting suitable for women?
There’s growing evidence that IF may affect women differently than men.
One study found that while IF improved insulin sensitivity in men, it actually worsened blood sugar control in women. Research in rats showed that prolonged intermittent fasting led to hormonal disruptions in females, including menstrual irregularities.
On the other hand, a clinical trial in women with PCOS found that 8-hour time-restricted feeding improved insulin resistance, reduced body fat, and helped regulate menstrual cycles in 73% of participants.7
The takeaway: women should approach IF more cautiously. Start gradually and stop immediately if you notice changes like missed periods. If you’re trying to conceive, pregnant, or breastfeeding, it’s best to hold off.
For a deeper dive, read our guide on intermittent fasting for women.
Summary: IF may not suit everyone, especially women with hormonal sensitivities. Start cautiously and consult a doctor if you have concerns.
Safety and side effects of intermittent fasting
Hunger is the main side effect, especially at the beginning. You might also feel weak or mentally foggy as your body adjusts. These effects are usually temporary — most people adapt within a few weeks.
Consult a doctor before starting IF if you:
- Have diabetes
- Struggle with blood sugar regulation
- Have low blood pressure
- Take medications
- Are underweight
- Have a history of eating disorders
- Are trying to conceive
- Have a history of amenorrhea (absent periods)
- Are pregnant or breastfeeding
For healthy, well-nourished people, intermittent fasting has an excellent safety profile. Going without food for a while is something our bodies are built to handle.
Learn more about potential intermittent fasting side effects.
Summary: Hunger is the most common side effect and usually fades. People with certain medical conditions should consult a doctor before starting.
Frequently asked questions about intermittent fasting
1. Can I drink liquids during the fast?
Yes. Water, coffee, tea, and other zero-calorie drinks are fine. Don’t add sugar. A small splash of milk or cream is usually okay. Coffee in particular can help manage hunger. Read more: Can you drink water when fasting? and coffee while intermittent fasting.
2. Is skipping breakfast unhealthy?
No — not on its own. The idea that breakfast is essential has been overstated. What matters is the overall quality of your diet. If the rest of your food intake is balanced and nutritious, skipping breakfast is perfectly fine.
3. Can I take supplements while fasting?
Yes. Keep in mind that fat-soluble vitamins (A, D, E, K) absorb better when taken with food.
4. Can I exercise while fasting?
Absolutely. Fasted workouts are safe for most people. Some people take branched-chain amino acids (BCAAs) before a fasted workout, though it’s not strictly necessary.
5. Will fasting cause muscle loss?
Any form of weight loss can involve some muscle loss. To minimize this, lift weights and keep protein intake high. Research suggests IF may actually cause less muscle loss than standard calorie restriction.4
6. Does fasting slow down metabolism?
No. Short-term fasting can actually boost your metabolic rate. However, extended fasts lasting three or more days may suppress it. Learn more about intermittent fasting and metabolism.
7. Should children fast?
No. Fasting is not recommended for children.
Also check out: What breaks a fast?
Getting started with intermittent fasting
You’ve probably fasted before without realizing it. If you’ve ever eaten dinner, slept, and waited until lunch the next day — that’s a 16+ hour fast right there.
The 16/8 method is the best starting point. It’s straightforward, flexible, and doesn’t require skipping entire days of eating.
Once you’re comfortable with 16/8, you can experiment with longer fasts if you want — like 24-hour fasts once or twice a week (eat-stop-eat) or the 5:2 approach.
Or keep it loose. Skip meals when you’re not hungry or don’t have time to cook. There’s no single “correct” way to fast.
The key is to find what fits your life and feels sustainable.
Summary: Start with the 16/8 method. Once comfortable, explore longer fasting periods if desired. The best approach is the one you can maintain.
Should you try intermittent fasting?
IF isn’t something everyone needs to do. It’s one tool among many for improving health.
The fundamentals still apply: eat real, whole foods, exercise regularly, and sleep well. Those matter more than any eating schedule.
If fasting doesn’t appeal to you, skip it. There’s no single best diet — the one that works is the one you’ll actually stick with long-term. Explore our best diet plans or learn about the keto diet for other popular options.
But if you’re curious, try it. Some people thrive on IF while others don’t. The only way to know is to give it a shot.
Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58. PubMed ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
Hjelholt A, Hogild M, Bak AM, et al. Growth Hormone and Obesity. Endocrinol Metab Clin North Am. 2020;49(2):239-250. PubMed ↩︎
Patikorn C, Roubal K, Veettil SK, et al. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. JAMA Netw Open. 2021;4(12):e2139558. PubMed ↩︎ ↩︎ ↩︎ ↩︎
Harris L, Hamilton S, Azevedo LB, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 2018;16(2):507-547. PubMed ↩︎ ↩︎
Zhang Q, Zhang C, Wang H, et al. Intermittent Fasting versus Continuous Calorie Restriction: Which Is Better for Weight Loss? Nutrients. 2022;14(9):1781. PubMed ↩︎
Jamshed H, Steger FL, Bryan DR, et al. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022;182(9):953-962. PubMed ↩︎
Li C, Xing C, Zhang J, Zhao H, Shi W, He B. Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. J Transl Med. 2021;19(1):148. PubMed ↩︎
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